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EGRET3: Help

EGRET3 - CGRO/EGRET Third Source Catalog


Overview

The Third EGRET Catalog of High-Energy Gamma-Ray Sources is based on data obtained by the Energetic Gamma-Ray Experiment Telescope (EGRET) on board the Compton Gamma-Ray Observatory (CGRO) during the period from 1991 April 22 to 1995 October 3, corresponding to GRO Cycles 1, 2, 3, and 4. EGRET is sensitive to photons in the energy range from about 30 MeV to over 20 GeV, the highest energies accessible by the CGRO instruments, and, like COMPTEL, is an imaging instrument. In addition to including more data than the Second EGRET Catalog (2EG, Thompson et al. 1995, ApJS, 101, 259) and its supplement (2EGS, Thompson et al. 1996, ApJS, 107, 227), this catalog uses completely reprocessed data so as to correct a number of mostly minimal errors and problems.

The 271 sources (E > 100 MeV) in the catalog include the single 1991 solar flare that was bright enough to detected as a source, the LMC, 5 pulsars, one probable radio galaxy detection (Cen A), and 66 high-confidence identifications of blazars (BL Lac objects, flat-spectrum radio quasars, or unidentified flat-spectrum radio sources). In addition, 27 lower-confidence potential blazar identifications are noted. Finally, the catalog contains 170 sources that are not yet firmly identified with known objects, although potential identifications have been suggested for a number of these.

As already noted, there are 271 distinct sources in this catalog: since there are multiple measurements for these sources corresponding to the various viewing periods, there are 5246 entries in the HEASARC's version of the 3rd EGRET Catalog corresponding to the same number of lines in Table 4 of the published version. Thus, there are an average of about 20 entries for every distinct source. Notice that 14 sources reported in the 2nd EGRET Catalog or its supplement do not appear in this 3rd EGRET Catalog: 2EG J0403+3357, 2EG J0426+6618, 2EGS J0500+5902, 2EGS J0552-1026, 2EG J1136-0414, 2EGS J1236-0416, 2EG J1239+0441, 2EG J1314+5151, 2EG J1430+5356, 2EG J1443-6040, 2EG J1631-2845, 2EG J1709-0350, 2EG J1815+2950, and 2EG J2027+1054 due to the fact that the re-analysis of the EGRET data has dropped their statistical significance from just above the catalog threshold to just below it; additional information on these sources is provided in Table 5 of the published version of the 3rd EGRET Catalog.


Catalog Bibcode

1999ApJS..123...79H

References

"The Third EGRET Catalog of High-Energy Gamma-Ray Sources", R.C. Hartman et al. 1999, ApJS, 123, 79.

Provenance

This database table was created by the HEASARC in June 1999, based on a machine-readable version of Table 4 of the 3rd EGRET Source Catalog that was provided by the CGRO Science Support Center (CGROSSC). Slight modifications to the Browse Object Classifications were later made in April 2001.

Data Products

Most, although not all, of the sources in the 3rd EGRET Catalog have two data products available: The first is a FITS file with suffix .fits, which contains the observed and best-fit model EGRET spectra of the source integrated over all the accumulated exposure time, together with the parameters of the best-fit model, while the second data product is a PostScript file with suffix .ps, which contains a plot of the data and best-fit model spectra. In general, the 3rd EGRET Catalog has multiple entries for any given source corresponding to different viewing periods, but often the individual observations have insufficient statistics to be of use spectrally, hence only the total exposure spectra have been generated. Thus, every entry (i.e., observation) of a given source has the same associated pair of data products, notice.

Parameters

Name
A Name based on the J2000 coordinates for the best position of the source, following the IAU naming convention (cf. PASP, 102, 1231). It should be emphasized that the measured positions are generally not as precise as the name might suggest.

RA
Right Ascension in the default equinox. The original data were provided in J2000 coordinates.

Dec
Declination in the default equinox. The original data were provided in J2000 coordinates.

LII
Galactic longitude.

BII
Galactic latitude.

Error_Radius
The radius in degrees of the circle containing the same solid angle as the 95% confidence contour, or, if the parameter error_radius_flag = 'I', by multiplying the 68% confidence radius by 1.62 (this latter was necessary in cases of unclosed or extremely irregular 95% contours).

Error_Radius_Flag
This parameter is set to the value of 'I' for sources for which the 95% confidence contour was unclosed or extremely irregular.

Flux
The flux (E> 100 MeV) in units of 10^-8 photons/cm^2/s, or upper limit thereof (for entries for whch the parameter flux_flag = '<', i.e., for those observations in which the source was not detected) in the same units. The flux for most sources was determined assuming a photon spectral index of 2. If the spectral index of a source differs significantly from 2, some additional error should be assumed for the flux.

Flux_Error
The 1-sigma statistical uncertainty in the flux. An additional systematic error of 10% should be added in quadrature to obtain the total error (this is important only for very strong detections).

Flux_Flag
This parameter is set to '<' for those observations in which a source was not detected.

Spect_Index
The photon number spectral index gamma where F(E) ~ E^{-gamma} as derived from the P1234 sum (which, in many cases, is not the most significant detection, notice). For those sources for which the P1234 sum does not give a sufficiently strong detection to derive a reliable spectral index, this parameter is left blank. spect_index_error = > The 1-sigma error in the spectral index.

Spect_Index_Error
Spectral index error

Counts
The number of E>100 MeV photons represented by the flux or upper limit thereof. The fractional uncertainty in the photon counts is the same as that of the flux, notice.

Sqrt_Ts
The statistical significance of the E>100 MeV detection. It is approximately equal to the statistical sigma for a single measurement at a fixed position.

Viewing_Period
The viewing period of the Observation. P1234 represents the sum of Cycles 1, 2, 3 and 4; 0.2+ is the sum of viewing periods 0.2 - 0.5, all of which covered roughly the same region of the sky during the verification phase of the mission, while other entries ending in + are for sums of 2-6 viewing periods and are explicitly defined in Table 6 of the published version of the 3rd EGRET Catalog (q.v.).

ID_Code
An identifier code, according to the following scheme:

     P = pulsar (indicates detection of pulsed gamma radiation)
     G = galaxy (the LMC is the only such object in the catalog)
     S = solar flare
     A = active galactic nucleus
     A:= possible active galactic nucleus.  This identification is questionable,
         either because the object has a low radio flux radio density (<300 mJy),
         or because it lies outside the 95% uncertainty contour, sometimes even
         outside the 99% uncertainty contour. Note that this is similar, albeit
         looser, than the notation of 'marginal' used in the 2nd EGRET Catalog
         and Supplement, but different from that used in the 1st EGRET catalog,
         which distinguished identifications by their statistical significance.
  

Alt_Name1
The source name from the 2nd EGRET catalog (2EG, Thompson et al. 1995, ApJS, 101, 259) or supplement (2EGS, Thompson et al. 1996, ApJS, 107, 227), or the 1st EGRET Catalog (1EG, Fichtel et al. 1994, ApJS, 94, 551) if not included in 2EG or 2EGS, or from other gamma-ray references. In some cases, the source position has moved from that given in earlier references. For identified sources, the name (or names) of the identified object may also be given in this field.

Alt_Name2
The source name from the 2nd EGRET catalog (2EG, Thompson et al. 1995, ApJS, 101, 259) or supplement (2EGS, Thompson et al. 1996, ApJS, 107, 227), or the 1st EGRET Catalog (1EG, Fichtel et al. 1994, ApJS, 94, 551) if not included in 2EG or 2EGS, or from other gamma-ray references. In some cases, the source position has moved from that given in earlier references. For identified sources, the name (or names) of the identified object may also be given in this field.

Alt_Name3
The source name from the 2nd EGRET catalog (2EG, Thompson et al. 1995, ApJS, 101, 259) or supplement (2EGS, Thompson et al. 1996, ApJS, 107, 227), or the 1st EGRET Catalog (1EG, Fichtel et al. 1994, ApJS, 94, 551) if not included in 2EG or 2EGS, or from other gamma-ray references. In some cases, the source position has moved from that given in earlier references. For identified sources, the name (or names) of the identified object may also be given in this field.

Alt_Name4
The source name from the 2nd EGRET catalog (2EG, Thompson et al. 1995, ApJS, 101, 259) or supplement (2EGS, Thompson et al. 1996, ApJS, 107, 227), or the 1st EGRET Catalog (1EG, Fichtel et al. 1994, ApJS, 94, 551) if not included in 2EG or 2EGS, or from other gamma-ray references. In some cases, the source position has moved from that given in earlier references. For identified sources, the name (or names) of the identified object may also be given in this field.

Alt_Name5
The source name from the 2nd EGRET catalog (2EG, Thompson et al. 1995, ApJS, 101, 259) or supplement (2EGS, Thompson et al. 1996, ApJS, 107, 227), or the 1st EGRET Catalog (1EG, Fichtel et al. 1994, ApJS, 94, 551) if not included in 2EG or 2EGS, or from other gamma-ray references. In some cases, the source position has moved from that given in earlier references. For identified sources, the name (or names) of the identified object may also be given in this field.

Refs
Previous EGRET references to the source, as identified in the footnote to Table 4 in the published version of the 3EG Catalog.

Redshift
The redshift of the counterpart object (only given for AGNs).

Note1
This parameter can have the following values, listed below with their meanings:

        @ =  See note in text of published version of 3EG.
        E =  Extended source (applies only to the LMC).
        EM = Possible extended source or multiple sources.
        C =  Source confusion may affect flux, significance, or position.
  
Sources with no entry (other than @) for any of the note parameters (note1, note2 and note3) are consistent with the EGRET point-spread function (PSF) for a single source.

Note2
This parameter can have the following values, listed below with their meanings:

        @ =  See note in text of published version of 3EG.
        E =  Extended source (applies only to the LMC).
        EM = Possible extended source or multiple sources.
        C =  Source confusion may affect flux, significance, or position.
  
Sources with no entry (other than @) for any of the note parameters (note1, note2 and note3) are consistent with the EGRET point-spread function (PSF) for a single source.

Note3
This parameter can have the following values, listed below with their meanings:

        @ =  See note in text of published version of 3EG.
        E =  Extended source (applies only to the LMC).
        EM = Possible extended source or multiple sources.
        C =  Source confusion may affect flux, significance, or position.
  
Sources with no entry (other than @) for any of the note parameters (note1, note2 and note3) are consistent with the EGRET point-spread function (PSF) for a single source.

Contact Person

Questions regarding the EGRET3 database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk
 
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